Loss of bone mass with aging has serious clinical consequences in terms of increased fracture incidence among the elderly. While changes with age in the histological and material properties of bone have been extensively investigated, age and sex-related differences in bone geometry have not been adequately documented. Individual geometric variation is also of clinical interest in the design of orthopaedic joint replacements and internal fracture fixation devices, which require precise geometric fitting and a favorable in-vivo stress environment to be successfully implemented. The specific aims of this study are threefold: 1) to provide normative cross-sectional geometric data for a large (n=80) sample of femora and tibiae, and to define sex and age differences in geometric properties; 2) to test hypotheses relating age changes in bone geometry to in-vivo mechanical stresses in the lower limb; and 3) to determine the effects of an endoprosthetic implant on bone remodeling in the proximal femur. Cross section geometric data will be obtained through direct sectioning and measurement of femora and tibiae collected from autopsy material. The sample will include 80 individuals and will be chosen to represent as equally as possible different sex and adult age groups. Twenty cross sections from each individual will be sampled, with a high concentration of sections in the proximal femoral diaphysis. Ten femora with in situ hip arthroplasty femoral components and their normal contralateral sides, also collected from autopsy material, will be used in the analysis of prosthesis-induced remodeling. Axial and bending stresses under theoretical loadings in both normal and prosthetic samples will be determined using three-dimensional beam theory, with torsional stress calculated using finite element analysis. Composite beam theory will be used to analyze geometric properties and stresses in compact-trabecular bone areas and in prosthesis-cement-bone composite sections. The relationships between stress distributions and geometric remodeling, both with age and following prosthesis implantation, will be examined in light of various proposed stress optimization theories.